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Postspreading seamounts are unique seamounts built by volcanism after seafloor spreading had ceased. Limited understanding of their formation processes is mainly due to the lack of investigation. Therefore, we conducted a three‐dimensional wide‐angle seismic experiment in the Southwest Subbasin of the South China Sea and four profiles were acquired within the inactive spreading center over a postspreading seamount. Based on ocean bottom seismometer data, we report two‐dimensional velocity structures of the Longnan seamount. Our results show that a low P wave velocity body (2.9–5.0 km/s, 4 km thick) is observed within the seamount summit, suggesting that material has extensively erupted as volcaniclastic rocks and high‐porosity basalts. The extrusive/intrusive boundary is depressed beneath the seamount, indicating the absence of an intrusive core. Our results further show that the ratio of extrusive to intrusive volume is as high as 3.0 and that the accretion to the oceanic crust occurs in layer 2. Combined with an analysis of velocity structures of seamounts worldwide including seamounts in the South China Sea, we propose that the extrusive/intrusive ratio and type of crustal thickening are related to the magma supply mechanisms, and intrusive cores are more likely to be associated with a hot spot. The above analysis also suggests that the Longnan Seamount is a nonplume origin seamount and structurally different from other seamounts in the South China Sea. Ridge jumps in the South China Sea increase the duration or amount of volcanism, which could explain seamount structural differences. Key Points: Studied seamount is extrusion dominated with an extrusive/intrusive ratio as high as 3.0 with nonplume originSeamount structures are mainly controlled by magma supply mechanismsAnomalous mantle due to southward ridge jumps could explain the difference of volcanism (and its structures) in the South China Sea [ABSTRACT FROM AUTHOR]

Abstract: Beneath ultraslow‐spreading ridges, the oceanic lithosphere remains poorly understood. Using recordings from a temporary array of ocean bottom seismometers, we here report an ~17‐days‐long microearthquake study on two segments (27 and 28) of the ultraslow‐spreading Southwest Indian Ridge (49.2° to 50.8°E). A total of 214 locatable microearthquakes are recorded; seismic activity appears to be concentrated within the west median valley at Segment 28 and adjacent nontransform discontinuities. Earthquakes reach a maximum depth of ~20 km beneath the seafloor, and they mainly occur in the mantle, implying a cold and thick brittle lithosphere. The relatively uniform brittle/ductile boundary beneath Segment 28 suggests that there is no focused melting in this region. The majority of earthquakes is located below the Moho interface, and a 5‐km‐thick aseismic zone is present beneath Segment 28 and adjacent nontransform discontinuities. At the Dragon Flag hydrothermal vent field along Segment 28, the presence of a detachment fault has been inferred from geomorphic features and seismic tomography. Our seismicity data show that this detachment fault deeply penetrates into the mantle with a steeply dipping (~65°) interface, and it appears to rotate to a lower angle in the upper crust, with ~55° of rollover. There is a virtual seismic gap beneath magmatic Segment 27, which may be connected to the presence of an axial magma chamber beneath the spreading center and focused melting; in this scenario, the increased magma supply produces a broad, elevated temperature environment, which suppresses earthquake generation. [ABSTRACT FROM AUTHOR]

Seismic azimuthal anisotropy characterized by shear wave splitting analyses using teleseismic XKS phases (including SKS, SKKS, and PKS) is widely employed to constrain the deformation field in the Earth's crust and mantle. Due to the near‐vertical incidence of the XKS arrivals, the resulting splitting parameters (fast polarization orientations and splitting times) have an excellent horizontal but poor vertical resolution, resulting in considerable ambiguities in the geodynamic interpretation of the measurements. Here we use P‐to‐S converted phases from the Moho and the 410‐ (d410) and 660‐km (d660) discontinuities to investigate anisotropy layering beneath Southern California. Similarities between the resulting splitting parameters from the XKS and P‐to‐S converted phases from the d660 suggest that the lower mantle beneath the study area is azimuthally isotropic. Similarly, significant azimuthal anisotropy is not present in the mantle transition zone on the basis of the consistency between the splitting parameters obtained using P‐to‐S converted phases from the d410 and d660. Crustal anisotropy measurements exhibit a mean splitting time of 0.2 ± 0.1 s and mostly NW‐SE fast orientations, which are significantly different from the dominantly E‐W fast orientations revealed using XKS and P‐to‐S conversions from the d410 and d660. Anisotropy measurements using shear waves with different depths of origin suggest that the Earth's upper mantle is the major anisotropic layer beneath Southern California. Additionally, this study demonstrates the effectiveness of applying a set of azimuthal anisotropy analysis techniques to reduce ambiguities in the depth of the source of the observed anisotropy. Key Points: Azimuthal anisotropy above the core‐mantle boundary, 660‐ and 410‐km discontinuities, and the Moho beneath Southern California are measuredSimilarities among splitting parameters using the three deepest discontinuities suggest insignificant anisotropy below the 410‐km discontinuityThe study demonstrates the effectiveness of investigating anisotropy layering using P‐to‐S conversions from interfaces at various depths [ABSTRACT FROM AUTHOR]